(1) Field of the Invention
The present invention relates to a water based intercoat coating composition showing excellent properties in a low solvent content or a low volatile organic compound content and in coating composition stability and capable of forming a coating film showing an excellent anti-chipping properties and to a multi-layer coating film-forming method by use of the water based intercoat coating composition.
(2) Description of Background Art
The automobile body is usually coated with a multi-layer coating film comprising an undercoat coating film with an electrodeposition coating composition, an intercoat coating film and a topcoat coating film for the purpose of imparting anti-corrosive properties and good appearance.
On the other hand, a coating composition using an organic solvent has mainly been used in the art. Since the use of the organic solvent may have possibility of danger of fire, influences on user's health, and recently influences particularly on environment, a coating composition using a safe and harmless solvent is demanded in the art.
In the field of the automobile coating, studies on a chemical treatment of substrate, a primer coating composition such as an electrodeposition coating composition, an intercoat coating composition and a topcoat coating composition for the purpose of preventing chipping and proceeding of corrosion due to the chipping have been made. Particularly, studies on the intercoat coating composition for the purpose of relaxing an impact force and preventing a peeling of a coating film from the surface of a metal, which is a cause of rust development have been made, resulting in demanding an intercoat coating composition having a function of the anti-chipping properties.
Recently, a tendency to thinning a film thickness and shortening a heat-curing time in the intercoat coating film from the standpoints of coating steps saving and energy saving demands an intercoat coating composition having an anti-chipping properties highly improved compared with the water based intercoat coating composition in the art.
Japanese Patent Application Laid-Open No. 256075/99 discloses an anti-chipping heat-curable powder coating composition prepared by adding a blocked polyisocyanate compound and a rubber particle and/or a crosslinked rubber particle comprising a rubbery polymer core layer and a vinyl polymer shell layer to a rubber-modified resin obtained by reacting a polyepoxide and a carboxyl group-containing rubber. Addition of the rubber particle and/or the crosslinked rubber particle showed poor properties in anti-chipping properties and coating composition stability under further severer conditions.
In addition to the intercoat coating composition capable of improving anti-chipping properties and making possible low VOC or reduction in volatile organic compound content, an intercoat coating composition capable of making possible good finished appearance and good coating composition stability is highly demanded.